I. Field of the Invention
This invention relates generally to timing circuitry for use in implantable medical devices, such as cardiac rhythm management devices, and more particularly to the design of a high frequency oscillator designed for use in such devices as cardiac pacemakers and automatic implantable cardiac defibrillators (AICDs) which is automatically retrimmed to compensate for frequency drift.
II. Discussion of the Prior Art
Many implantable medical devices (such AICDs) require a high-speed oscillator, operating in the megahertz range, to time a high-speed microprocessor, to run telemetry circuitry within the implanted device and to function as a redundant oscillator for fault detection purposes. These devices will also commonly utilize a low speed oscillator, for example, one operating in the 32 KHz range for timing operations. Given the application in cardiac rhythm management devices that are implanted within the body, the high-speed oscillator used in such devices have several unique requirements. First, the high-speed oscillator must be redundant and must operate independently of the relatively low speed oscillator. A redundant high-speed oscillator allows for effective fault detection in the event that the low speed oscillator should fail in the field. Another requirement for the high-speed oscillator is that it must have a rapid start-up time, providing a clock output within microseconds of its being enabled. Because of size constraints, the high-speed oscillator should have a minimal component count and be conservative of battery power, preferably operating in a microwatt range. Finally, the high-speed oscillator must be stable and as accurate as possible.
In the past, RC oscillators have been used in implementing the high-speed oscillator used in pacemakers and AICDs, principally because the requirements for a fast start-up time and for minimum component count has precluded the use of a crystal-controlled high-speed oscillator. Furthermore, the requirement that the high-speed oscillator operate independent of the low-speed oscillator has prevented the use of a phase-lock loop design. The prior art RC oscillator typically utilize a laser trimmable resistor which, at the time of manufacture, is trimmed so that the high-speed oscillator will produce a desired output frequency, e.g., about 2 MHz. However, this technique does not always produce reliable results in that associated with the external resistor is stray capacitance and trimming of the resistor is found to vary the stray capacitance. This makes it difficult to accurately determine the trimmed resistance value needed. Likewise, where the circuitry is to be encapsulated following the laser-trimming of the resistor, such encapsulation is found to also vary the capacitance across the resistor which, of course, resulted in a change in frequency of the high frequency oscillator from its trimmed value.
It is accordingly a principal object of the present invention to provide an improved high frequency oscillator for implantable medical devices.
Another object of the invention is to provide an improved high frequency oscillator that is automatically retrimmed each time it is enabled to thereby compensate for drift due to temperature changes, noise or component aging.
It is a further object of the present invention to provide a high frequency oscillator for use in implantable medical devices that can be rapidly activated and that automatically undergoes retrimming each time it is enabled to provide improved frequency stability with less components and with lower current drain.
The foregoing objects and advantages of the invention are realized by providing a timing circuit for an implantable cardiac rhythm management device that comprises a crystal-controlled oscillator for producing an output signal of a predetermined frequency, f1, a second oscillator for producing an output signal of a relatively high frequency, f2, where f2xe2x89xa7f1. The two oscillators each provide an input to a frequency comparator that produces an output that varies proportional to any deviation of the frequency, f2, relative to the stable crystal controlled frequency, f1. The output of the frequency comparator is used in a feedback arrangement to trim the frequency of the non-crystal controlled oscillator to compensate for the deviation, whereby the frequency stability of the non-crystal controlled oscillator is maintained.
The timing circuit further comprises an oscillator fault detector that provides an indication when the deviation of f2 relative to f1 falls outside of predetermined limits.